1. Technical Field of the Invention
The present invention relates generally to a radar such as an FMCW (Frequency Modulated Continuous Wave) radar which is designed to transmit a frequency-modulated radar wave and receive a return thereof from an object through a plurality of antennas to determine the distance to, relative speed, and azimuth or angular direction of the object.
2. Background Art
Monopulse radars are known as automotive radars. For example, U.S. Pat. No. 5,757,307 discloses such a radar. The monopulse radar works to receive a radar echo from a reflective object through two antennas arrayed at a given interval away from each other and determine an azimuth or angular direction of the object based on a phase difference between signals received by the antennas. The principle of determining the angular direction will be described below in brief.
Consider a radar system, as illustrated in FIG. 13, in which two antennas A1 and A2 which are located at an interval D away from each other receive a radar return which has a wave length λ and has been reflected from an object M existing in a direction making an angle θ with a line extending perpendicular to planes of the antennas A1 and A2. Paths along which the radar return travels from the object M to the respective antennas A1 and A2 are different in length by a distance Δd. The path length difference Δd depends upon the angle θ which the line extending perpendicular to the planes of the antennas A1 and A2 makes with the direction of the radar return. The angular direction (i.e., the angle θ) of the object M may, thus, be determined using a phase difference Δφ between the signals received by the antennas A1 and A2 which may be considered as the path length difference Δd. The relation between the phase difference Δφ and the angle θ is given by the following equation:Δφ=(2π/λ)D sin θ  (1)
When the angle θ is small sufficiently, sin θ may be considered to be equal to θ (i.e., sin θ≈θ). Eq. (1) may be rewritten, as shown below, in terms of the angle θ.θ=Δφ·λ/(2π·D)  (2)
How to determine the phase difference a Δφ will be described below.
The radar system works to transmit a triangular wave radar signal which is frequency-modulated to have a frequency increasing and decreasing, i.e., sweeping upward and downward cyclically in a linear fashion and receive a return of the transmitted radar signal from the object M through the antennas A1 and A2. The radar system mixes portions of a signal received by each of the antennas A1 and A2 within ranges where the frequency of the transmitted signal sweeps upward and downward (will also be referred to as modulated frequency-rising and -falling ranges below) with the transmitted signal to produce frequency signals whose frequencies are equal to differences in frequency between the portions of the received signal and the transmitted signal (will also be referred to as a frequency-rising range beat signal and a frequency-falling range beat signal below). Note that the modulated frequency-rising and -falling ranges are also called an up-chirp and a down-chirp, respectively. Next, the radar system samples the frequency-rising and -falling range beat signals in sequence and subjects them to Fast Fourier Transform (FFT) to produce frequency spectra thereof in the modulated frequency-rising and -falling ranges. The frequency spectra are derived as complex vectors in each of a series of frequencies.
The radar system searches frequency peaks from absolute values of the complex vectors in each of the modulated frequency-rising and -falling ranges. The frequency peaks in the modulated frequency-rising and -falling ranges arise from a radar return from a reflective object and depend upon the distance to and relative speed of the object. Next, the radar system determines phases of the beat signals at the frequency peaks in the modulated frequency-rising and -falling ranges. The phases are each derived by, for example, the angle which the complex vector makes with a real number axis. The radar system finds a difference in phase between the frequency peaks of the received signals in the modulated frequency-rising range and also a difference in phase between the frequency peaks of the received signals in the modulated frequency-falling range.
The radar system elects one of the phase differences in the modulated frequency-rising and -falling ranges as the phase difference Δφ and determines the angular direction of the object M according to Eq. (2), as described above.
The frequency peaks in the modulated frequency-rising and -falling ranges are also used to determine the distance to and the relative speed of the object M. This will be described below with reference to FIGS. 14(a) and 14(b).
When an automotive vehicle equipped with a radar is traveling at the same speed as that of a reflective object, that is, when the relative speed V of the object is zero (0), a radar return from the object undergoes a time lag equivalent to a time interval between transmission of a radar wave and reception the echo (i.e., a time required for the radar wave to travel twice the distance D between the radar and the object). Specifically, as illustrated in FIG. 14(a), a radar-received signal fr is shifted in the time domain by such a time lag from a transmit signal fs, so that the frequency peak fbu in the modulated frequency-rising range (will also be referred to as an upward frequency peak below) will be identical with the frequency peak fbd in the modulated frequency-falling range (will also be referred to as a downward frequency peak below).
When the radar-equipped vehicle is traveling at a speed different from that of the object, that is, when the relative speed V is not zero (0), it will cause a radar return from the object to undergo a Doppler shift as a function of the relative speed V between the radar-equipped vehicle and the object. The received signal fr is, thus, shifted in frequency by an amount corresponding to the Doppler shift as well as the time lag that is, as described above, a function of the distance D to the object. This causes, as shown in FIGS. 14(a) and 14(b), the upward frequency peak fbu to be different from the downward frequency peak fbd (fbu≠fbd).
Specifically, the received signal fr is shifted in the time and frequency domains as functions of the distance D to and the relative speed V of the object. In other words, a difference in frequency between the transmitted signal fs and the received signal fr in the time domain (will also be referred to as a frequency fb below) is a function of the distance D to the object, while a difference in frequency therebetween in the frequency domain (will also be referred to as a frequency fd below) is a function of the relative speed V of the object. The frequencies fb and fd are expressed below.fb=(|fbu|+|fbd|)/2  (3)fd=(|fbu|−|fbd|)/2  (4)
Using the frequencies fb and fd, the distance D to and relative speed V of the object may be expressed as:
 D={C/(4×ΔF×fm)}×fb  (5)V={C/(2×fo)}×fd  (6)where ΔF is a variation in frequency (i.e., amplitude) of the transmitted signal fs, fo is a central frequency of the transmitted signal fs, fm is a modulating frequency of the transmitted signal fs, and c is the velocity of light.
If there are a plurality of objects reflecting a radar wave, as many upward frequency peaks fbu and downward frequency peaks fbd as the objects appear. Determination of the distance D to and the relative speed V of each object, thus requires pairing of each of the upward frequency peaks fbu with a corresponding one of the downward frequency peaks fbd. For example, ones of the upward frequency peaks fbu and the downward frequency peaks fbd which have phase differences Δφ close to each other may be paired. This is based on the fact that combinations of the upward and downward frequency peaks fbu and fbd having close phase differences Δφ may be considered to have arisen from the same objects, respectively.
However, if frequency peaks arising from a plurality of objects overlap each other, it will cause composite phases to be derived each of which is a mix of phases of radar returns from the objects, thus resulting in a difficulty in pairing the upward and downward frequency peaks fbu and fbd correctly, which leads to errors in determining the distance to, angular direction, and relative speed of each object.